There presently exist many technologies for forming of sinterable powders such as metals, ceramics, carbides, oxides, hydrides and the like via binder assisted forming methods. Sinterable powders can be admixed with temporary binder compositions and formed by a variety of forming techniques including pressure molding, injection molding, and extrusion. The admixtures of powder and binder are generally referred to as feedstock materials or compositions. After being formed into a shaped body or part, the binder is removed and the powders are sintered to produce the final product. This final product may have open porosity, closed porosity, or be completely dense.
The binder generally comprises polymers and other organic chemicals. Various methods can be employed to remove the binder from the part prior to or during sintering or other processing operations. Some binders are completely removed by thermal methods in a controlled fashion, others have multiple phases or components that are removed sequentially by extraction, catalytic decomposition, or thermal treatment.
Binder removal is frequently referred to as debinding. Multi phase binders frequently include an extractable phase, which is removed first, and a backbone phase that is removed after the extractable phase. The extractable phase is usually a material that can be conveniently removed by non-thermal means such as a solvent extraction or a catalytic decomposition method. The solvent is typically an organic solvent or water. Additionally, the extractable phase may be removed thermally by evaporation or sublimation. After removal of the extractable phase the remaining binder backbone components are generally thermally decomposed leaving the part or body essentially free of all of the temporary binder materials or components.
Many formulations use a relatively substantially high molecular weight polymer as the backbone phase and a lower molecular weight material such as a wax or similar lower molecular weight material or chemical as the extractable phase. The backbone polymer provides strength necessary to allow forming and subsequent handling and processing of the molded body. The lower molecular weight extractable phase materials are used to plasticize the backbone phase. This imparts much lower viscosity to the feedstock, powder plus binder, and allows parts to be more easily formed. The extractable phase is typically removed via solvent immersion and lower molecular weight materials are more readily removed by this technique.
There are multitudes of binder compositions that reflect this general formulation strategy. A common binder system formulation uses a polypropylene or polyethylene as a backbone and a wax as the extractable phase. Other systems have used polymethylmethacrylate, polyoxymethylene, polyvinyl butyral or a phenoxy resin as the backbone and polyethylene glycol as the extractable phase. Other systems have used polystyrene as the backbone and mixture of oils or waxes as the extractable phases.
These systems all have in common the use of a much lower molecular weight extractable phase to plasticize the higher molecular weight backbone phase. There are two main detriments to this approach. The first is that the extreme plasticization of the backbone by a much lower molecular weight material greatly weakens the overall strength of the binder. The second is that even though the viscosity has been lowered drastically, the backbone molecules are still long enough to require high shear rates to achieve a usable viscosity for injection molding. Feedstock formulated with approach will exhibit significant shear thinning behavior.
Other formulations use a catalytic debinding approach. This approach catalytically decomposes the extractable phase, essentially reverting the polymer to its monomer or other relatively small, volatile molecules. Higher molecular weights can be used and these binders typically have very high green strengths. Commercially available systems incorporating this design use polyoxymethylene as the extractable phase and a polyolefin as the backbone. Because of their high molecular weight these binders exhibit shear-thinning behavior.
Another formulation approach is to use a gelation type binder. The extractable phase in this case is most often water. Often a polysaccharide or cellulose based material is used as the backbone. The materials are combined to form a gel, which when subjected to shear during injection, breaks up to allow flow. After injection the material gels again to form the molded article. The extractable phase (water) is then removed by a drying step. Gelation binders such as these rely on a shear thinning mechanism during the transition from rigid gel to moldable material. These formulations exhibit a lack of strength out of the mold and process control issues related to evaporation of the extractable phase prior to processing.
All of these designs are approaches to forming sinterable powders. In practice, the injection molding of powders is complicated by many processing details related to avoiding the initiation of flaws during the forming and subsequent debinding and sintering of parts. One of the main challenges is the injection molding process itself. Because of their shear-thinning behavior, these formulations all require high shear rates to achieve a sufficiently low viscosity during molding. High shear rates are achieved by using high injection speeds. Many of the challenges presented by the injection molding process are rooted in the use of binders requiring high shear rates to achieve low viscosities and the high speeds that must accompany these high shear rates.
When a binder is subjected to high shear rates the material can be rapidly overheated in isolated spots throughout the bulk. Shear heating such as this can be very detrimental to the components of the binder system. Lower molecular weight materials such as additives or plasticizers can easily be volatilized when subjected to shear heating. During processing significant internal vapor pressure can be generated due to the shear heating of these more volatile components. These pressures, combined with the low strength of the binder in its molten or partially solidified state, can result in defects such as surface blisters or internal delaminations. Shear heating is not only detrimental to the lower molecular weight additives in a formulation, but also to the polymeric materials which give a formulation its strength. Shear heating can degrade the polymeric molecules, breaking the polymers down into lower molecular weight materials of different viscosities and strengths. This not only yields a part with compromised binder components, but also aggravates the differences between virgin material and material that has previously been molded, presenting a difficulty to the processor.
A second general set of consequences resulting from the speed used to shear thin prior art materials is rooted in the density differences between the powder being formed and the binder being used to form it. When a powder binder mixture is injection molded it must often be subjected to changes in direction. Because of the inertial differences between the powder and the binder, the powder will tend to resist directional changes more than the binder, resulting in an undesirable separation of powder and binder.
Another consequence of shear thinning binders and the concomitant high injection speeds is the difficulty of establishing a controlled melt front. Many molded parts are desired to be free of internal voids. In order to mold a void free part a controlled melt front must be established to allow the part to fill without the melt jetting or splashing around in the mold cavity. As soon as jetting occurs, air can be entrapped in the melt and it becomes very difficult to mold a void free part. By avoiding high speeds the melt front can be much more easily controlled and void free parts molded. Also, the challenges of molding a large void free part can be more easily overcome because of the ease of controlling a low speed melt front.
The practical implications of the need for high shear rate due to the shear thinning nature of prior art binders are presented not only by processing concerns regarding the feedstock itself but also by special requirements of processing equipment.
The previous discussions have addressed the problems encountered in the feedstock or molded part when using high shear rates to injection mold. Other problems arise from the process rather than the product side. High injection rates require much higher degrees of control in the injection molding equipment. Prior to the filling of the mold cavity, the injection rate must be slowed down to allow for flash free parts. If the cavity fills at too high of a speed, the melt slams into the parting lines, slides, and vents of the mold, creating undesirable flash and cleaning issues. The use of high injection rates creates the need for high speed controls, which increases the cost of the manufacturing equipment. Additionally, in practice high speed controls may still be inadequate to eliminate the discussed problems.
The use of binders that incorporate a shear thinning mechanism to achieve low viscosity is fraught with many technical problems. These problems are rooted in the high speeds needed shear thin the binders and allow the powder/binder mixture to be molded. Some formulations have low shear-thinning behavior due to the inclusion of large amount of low molecular weight wax. These systems are extremely weak because of the low strength of the waxes. Low strength greatly impedes molding, handling and processing.
It is an object of this invention to provide a process to formulate a multiple phase binder for sinterable powder in such a manner that shear thinning behavior is reduced or eliminated in order to allow powder/binder mixture to demonstrate low viscosities at low shear rates. This is achieved by the tailoring of the molecular weight and the molecular weight distribution of the polymers constituting the extractable and backbone phases of the binder. It is another object of this invention to provide a process to formulate binders of high strength.
The concept of tailoring the molecular weight of the binder phases to provide these more desirable flow properties can be applied to many different binder chemistries. Many formulations have been concerned primarily with the debinding mechanism. Flow properties were achieved secondarily by the addition of plasticizers or surfactants. A binder designed from this approach is severely limited because the incorporation of plasticizers tends to weaken the polymer matrix, which is already severely weakened by the inclusion of 45 to 75 vol. % sinterable powder. Also, because the flow behavior has not been considered from the onset, the debinding mechanism, however clever, is still used in the context of a shear thinning binder system.
In order to allow the extraction of one phase from another, the phases need to be chemically different from one another. FIG. 1 depicts an interface between these different phases. 1.1 represents the backbone phase of the binder and 1.2 represents the extractable phase. The space between them (1.3) represents the interface. Because of the chemical differences between phases there also exists an interfacial tension between the polymeric phases of a binder. Reducing this tension will reduce the overall viscosity of the binder system and allow it to be more easily formed. Previous work has addressed reducing this interfacial tension through the use of certain chemicals as compatibilizers. For example, U.S. Pat. No. 5,641,920 to Hens cites the use of a small molecule such as monoglycerol monostearate to compatibilize and also plasticize the binder. This approach can achieve lower viscosity by introducing a third phase that is compatible to some extent with both major phases; this molecule allows slippage of the phases by one another by introducing a mutually compatible chemical at the phase interface.
The difficulty with using chemical compatibilizers is that the placement of a small molecule between the phases of the macro-molecular matrix significantly compromises the overall strength of the binder. While a chemical compatibilizer may have an affinity for both binder phases and can reside at the interface of these phases, it lacks the mechanical length to extend into the phases. By residing at the interface they reduce the tension between the phases but also reduce the strength of the binder by introducing a non-interpenetrating layer between binder phases. FIG. 2 illustrates a phase interface in a binder system incorporating a chemical compatibilizer. The backbone phase is represented by 2.1, and the extractable phase is represented by 2.2. The space between these phases (2.3) represents the phase interface. The chemical compatibilizer molecules are represented as 2.4. These molecules reside at the phase interface but do not extend significantly into either phase.
Chemical compatibilizers are short molecules that can have a significant vapor pressure at the processing temperatures. In addition to direct temperature affects, heating due to the shear present in binder assisted forming processes can also contribute to the volatilization of these molecules. During processing significant internal vapor pressure can be generated due to the inclusion of these prior art chemical compatibilizers. At many points during processing the binder is in a molten or partially solidified state. At these points the strength of the binder is very low and even a slight internal pressure can exceed the strength of the binder and cause defects. The combination of reduced strength due to the interfacial presence of these chemicals and their volatile nature can create a situation where blistering and delaminating defects are very easily created. These agents are also cited as plasticizers. While this may reduce the melt viscosity, chemical plasticization of this nature reduces the mechanical properties of the binder system.
An object of this invention is to reduce the interfacial tension between binder phases without compromising the integrity of the binder system by incorporating a polymeric compatibilizer into the binder. This polymeric compatibilizer is a macromolecule containing multiple segments of different chemical natures which are compatible with the different binder phases. A further object of this invention is to use polymeric compatibilizers in polymer alloys to allow for improved mechanical properties of molded components. The use of these materials in filled and unfilled polymer alloys is well documented. However, these applications were intended to optimize the mechanical performance of the molded parts. The prior art does not teach their application in multiphase binders incorporating an extractable phase, nor does it envision their application in the temporary binding of powders. Additionally, the prior art also does not address the application of polymeric compatibilizers for the improvement of the rheological (as opposed to mechanical) behavior of binder formulations for the binder assisted forming of sinterable powders. It is an object of this invention to provide a process to improve the processing behavior of multi-phase binder systems for sinterable powders by reducing the interfacial tension between the phases of the binder system without compromising the performance of the binder system.
An additional challenge to the formation of defect free parts formed using a multiple phase temporary binder is the inherent incompatibility of inorganic powder surfaces with most polymers. Because of these differences it is difficult for the powders to be wetted by the polymeric elements of the binders. This problem is conventionally solved by using a small organic or organometallic molecule with an affinity for both the powder and the binder. Typically one end of these molecules is compatible with the powder surface and the other end compatible with the polymer binder. The molecule orients itself along the powder/binder interface and allows the polymer to wet the particle. There are many names for these molecules, but in this application they all serve a similar purpose. Among the descriptors for molecules acting in this capacity are: surfactant, dispersant, surface active agent and coupling agent. All serve to reduce the interfacial tension between the powder and the binder in which it is dispersed. There are many examples of the use these materials and their application is known to those skilled in the art.
A common attribute of these materials is that the molecules are very small relative to the polymeric binder in which they are used. This is advantageous because the powders are often pretreated with the surfactant prior to being mixed with the polymeric binder and the small size of the molecules allows easy dispersion into the powder because it can generally easily be dissolved in a solvent. Also there are a great many materials available which can perform this function. One of the most prevalent materials used in this application is stearic acid. The carboxylic functionality is polar and is attracted to the hydrophilic powder surfaces. The stearyl tail is hydrophobic and is more compatible with the polymer binder. Glycerol stearate and many other molecules are also used as a surfactant in a similar manner.
Surfactants like these are well suited for use as dispersants in slurries and other low viscosity, low molecular weight mixtures. They have been adapted to polymer blends with some success but they face inherent limitations due to their small size. The short length of the section of the molecule that is compatible with the binder does allow better dispersion of the powder into the binder system by reducing the interfacial tension at the powder binder interface. However, it does not extend significantly into the polymeric matrix. The interfacial tension is reduced, but the link between the two phases, powder and binder, is not especially strong. Composite strength gains are made, but mostly by decreasing powder agglomeration and improving wetting, not by integration of the surfactant into the polymer matrix. Some organometallic molecules, silane compounds in particular have been essentially grafted into the polymer backbone during melt processing. By extending the surfactants reach by joining surfactant to the polymer bulk the mechanical properties are greatly improved because powder/binder interface is better bridged. Because this approach is concerned primarily with mechanical properties and not Theological properties, it has found most of its successes in the field of filled polymers or composites rather than temporary binders for sinterable powders.
Another concern regarding the degree of mechanical extension of the surfactant into the binder system is separation of the powder from the binder during injection molding. Because the surfactant does not extend significantly into the binder it is of limited usefulness in preventing powder binder separation due to inertial differences.
Aside from mechanical extension into the polymer binder, chemical compatibility is also a concern. Although a short organic chain group is much more compatible with a polymer than an inorganic surface, it is by no means necessarily the best choice for compatibility with a polymer. A material's success as surfactant is based on its relative compatibility to the polymeric binders rather than optimum compatibility.
Another concern is that any excess surfactant that is not tied up at the powder/binder interface will end up distributed through out the binder. If the binder is subjected to shear heating, as it will be in most forming processes, these excess very low molecular weight materials can be volatilized, creating a vapor pressure that can cause defects during the forming process.
It is an object of this invention to use polymers as surfactants in temporary binders for the binder assisted forming of sinterable powders. The use of a higher molecular weight material as a surfactant will allow better chemical and mechanical integration of the surfactant into the binder, will reduce the presence of molecules with any considerable vapor pressure and will help the powder binder mixture to resist separation during forming. A further object of this invention is to provide an improved method for predispersing these macromolecules into the powder prior to the compounding of the powder/binder mixture. Another object of this invention is to combine these macromolecular surfactants with conventional surfactants to provide optimum rheological characteristics.
This invention allows the problems presented by the use of conventional surfactant molecules to be avoided while at the same time solving the problems that required the use of the conventional surfactant in the first place. The conventional surfactant is replaced by a macromolecular surfactant.